Alumina trihydrate (Al2O3.3H2O) is produced by the well known Bayer process and then heated in a multi-stage calciner to make smelting grade alumina (SGA) having low water content. Airborne dust produced during calcination is collected by means of electrostatic precipitators and in multi-cyclones and bag houses. The dust thereby collected is a very fine alumina having a particle size which varies considerably. Water content is low, generally less than about 3 wt %. Typically more than 90 wt % of the dust is in the form of particles less than 20 microns in size and the particles generally have a median size of less than about 5 microns. Because of its small particle size the dust is difficult to handle with equipment ordinarily used for feeding alumina into smelting cells. Accordingly it is impractical to add large amounts of dust directly into an electrolytic cell. Alumina-containing dust collected by electrostatic precipitators is sometimes called “ESP dust”.
In the prior art several processes have been devised for agglomerating ESP dust into bodies suitable for addition to electrolytic cells for aluminum production. References relating to such processes include Bhilotra U.S. Pat. No. 4,075,067; Thé et al. U.S. Pat. No. 5,296,177; Boulanger U.S. Pat. No. 5,560,876; Hall et al. U.S. Pat. No. 5,858,325; and Robson et al. U.S. 2005/0118096. Disclosures of the Bhilotra, Thé, and Boulanger patents are incorporated by reference to the extent consistent with the present invention. Processes disclosed in the cited references are not in widespread industrial use at the present time because they rely upon expensive binding additives such as activated alumina or they are otherwise impractical to operate commercially. Accordingly there still remains a need for an efficient and economical process capable of agglomerating ESP dust into bodies suitable for addition to electrolytic smelting cells for producing aluminum.
A principal objective of the present invention is to provide a process and apparatus suitable for agglomerating alumina-containing dust into bodies suitable for addition to electrolytic smelting cells. A related objective of the invention is to provide an efficient and economical process for aluminum production utilizing alumina-containing dust as a starting material. An important advantage of the invention is that the process relies upon agglomerating the alumina-containing dust with addition of an inexpensive binder. Additional objectives and advantages of my invention will become apparent to persons skilled in the art from the following detailed description of a particularly preferred embodiment.